Effects of foliar fertilizer application on the growth and fruit quality of commercial melon varieties grown in a soilless culture system

Several factors influence the quality of melon fruits and foliar fertilizer application is one method for improving their quality. The objectives of this study were: (1) to investigate the response of commercial melon varieties to a soilless culture system in Nakhon Si Thammarat Province, Thailand, and (2) to evaluate the quality of melon fruit under various foliar fertilizer treatments. The experiment was arranged as a completely randomized block design with four replications. Eight commercial melon varieties, including four orange pulp melons (Sandee, Baramee, Sanwan, and Melon cat 697) and four green pulp melons (Kissme, Snowgreen, Melon Princess, and Kimoji), were used in this study. At 1–5 weeks after planting, the growth of the melons was measured using agronomic traits. Four foliar fertilizers (distilled water, micronutrients, secondary nutrients + micronutrients, and amino acid + micronutrients) were sprayed on the melon leaves at 1–5 weeks after pollination, and the growth of the melons, using fruit traits, was recorded. After harvesting, the melons were assessed for the quality of the fruit. This study was conducted at the School of Agricultural Technology and Food Industry’s greenhouse and the Food Chemistry Laboratory of the Center for Scientific and Technological Equipment, Walailak University. In nearly all of the observed growth weeks, the data demonstrate that most agronomic and fruit traits were significantly different between the melon varieties. Sandee, Baramee, Melon cat 697, and Melon Princess are recommended for planting under Nakhon Si Thammarat’s climate, based on fruit size and quality. Foliar fertilizer application impacted the shape, skin color, and quality of the melon. Melons treated with micronutrients, secondary nutrients and micronutrients, and amino acids and micronutrients exhibited better measures of fruit quality than those treated with non-foliar treatments. There was also an interaction observed between melon variety and foliar fertilizer application. Based on measures of fruit quality, Baramee, Melon cat 697, Kissme, and Melon Princess were more responsive to foliar fertilizer application than other melon varieties tested.


INTRODUCTION
Melon, or sweet melon (Cucumis melo L.), a member of the Cucurbitaceae family, is one of the world's most well-known fruit crops and is an excellent source of phytonutrients such as cucurbitacin, lithium, and zinc (Yavuz et al., 2021). Because of their high antioxidant content, anti-inflammatory qualities, antidiabetic advantages, antiulcerogenic activity, and antibacterial capabilities, melons have significant human health benefits (Bianchi et al., 2016). Melon fruit production depends on several factors such as variety, growing season, soil property, pollination, irrigation, and fertilization (Nerson, 2011;Martuscelli et al., 2015). Fertilization is critical to melon fruit quality. Several studies have found that a lack or imbalance of plant nutrients causes melon fruit quality to deteriorate. For example, cracked melons are associated with numerous elements including K, P, Ca, Mg, Mn, Na, B, and Zn (Lopez-Zaplana et al., 2020). Some of the fruit quality attributes of melons, such as fruit firmness, pulp thickness, rind thickness, and TSS, are influenced by nitrogen associated with phosphorus (Bouzo, Céccoli & Muñoz, 2018).
Foliar fertilizer application, where plant minerals are dissolved in a solution and sprayed directly onto plant leaves, is a technique used to improve fruit quality and productivity in a variety of horticulture crops (Muñoz, Ruiz & Bouzo, 2017). Minerals are absorbed through the stomata and epidermis of the leaves (Patil & Chetan, 2018). However, because this procedure may be detrimental to plant leaves, it is only recommended for low-concentration fertilizers. Foliar fertilization and spraying are regularly used in melon cultivation. In one study, the appearance of melon fruits significantly improved following the foliar fertigation of several micronutrients (Lopez-Zaplana et al., 2020). Early flowering, fruit number, and total yield are all promoted by the foliar fertigation of potassium and calcium fertilizers alone, or in combination with borax (Srilatha, Padmodaya & Kumar, 2017). In another study, the foliar spray of some plant growth stimulants, such as plant amino acid and yeast extract, along with nitrogen fertilization, improved some melon plant growth characteristics (plant length, leaf area, weight of shoots per plant) and fruit qualities (fruit diameter, fruit weight, fresh yield, nutrient uptake, total sugar, and TSS) (Morsy, Abdel-Salam & Shams, 2018). A soilless culture system is a method of growing plants which does not require the use of soil as a growing medium. Plants acquire nutrients by irrigation water, which is referred to as "nutrient solution." The advantages of a soilless culture system include a reduction or elimination of soil-borne diseases, the ability to control the amount of water and nutrients used, and an increase in nutrient availability (Putra & Yuliando, 2015). This technique is utilized in a wide range of vegetable and horticultural crops, such as lettuce, strawberry, tomato, and melon (Savvas & Gruda, 2018). Melons planted in a soilless culture system have higher values of stem height, leaf number per plant, total leaf number, fruit per plant, fruit weight, and total fruit yield, than melons planted using soil cultivation (Singer et al., 2013). High quality melon fruit production requires appropriate nutrient management. It is crucial for melon cultivation in Southern Thailand to determine the best foliar fertilizer treatment for various melon varieties.
The objectives of this study were: (1) to investigate the response of commercial melon varieties to a soilless culture system in Nakhon Si Thammarat Province, Thailand and (2) to evaluate the quality of melon fruit under various foliar fertilizer applications.
The experiment was conducted between November 2021 and February 2022. During the experiment, the temperature ranged between 21.5 C and 33.9 C, with February as the hottest month. Monthly rainfall ranged between 27.6 and 446.8 mm, and the relative humidity ranged between 84 and 93 percent, with the highest amount of rainfall and humidity occurring in November, and the highest relative humidity in February. The weather data was recorded by the greenhouse's weather station.

Data collection
Plant growth and flowering characteristics, including stem diameter (measured at the middle of stem), stem length, leaf width, leaf length, leaf number, days to male flowering, and days to female flowering, were recorded 1-5 weeks after transplanting. Fruit growth characteristics, including fruit weight, fruit height, and fruit perimeter, were recorded 1-6 weeks after pollination. Ripe melons were harvested at 6 to 7 weeks after pollination for the fruit quality assessment. The fruits were measured and analyzed for fruit cavity size, pulp thickness, rind thickness, firmness, total soluble solid (TSS), juice electrical conductivity (EC), juice pH, titratable acidity (TA), and pulp color (L Ã , a Ã , b Ã , hue, and chroma (c Ã )). A portable Hunterlab ColorFlex Ò EZ device (Hunter Associates Laboratory, Reston, VA, USA) was used to collect colorimetric data. A white and black standard were used to calibrate the device. The following measurement modes were chosen: tristimulus L Ã (lightness), a Ã (redness/greenness), and b Ã (yellowness/blueness). The L Ã -axis measures lightness on a scale of 0 (black) to 100 (white). The a Ã -axis goes from green (−a Ã ) to red (+a Ã ), while the b Ã -axis runs from blue (−b Ã ) to yellow (+b Ã ). Hue is defined as the color appearance parameters of a color, also known as color tone or color name. Chroma is a color's saturation level, with possible chroma values: clear, bright, or brilliant.
A Brix refractometer (model RHB-18ATC; Sinotech Ltd., Watthana, Thailand) was used to measure the TSS, with results expressed as Brix. TSS measures the balance of sugars and acids in a matrix, which affects fruit flavor (Fundo et al., 2018). EC and pH meters (edgeÒ model HI2030; Hanna Instruments, Inc., Woonsocket, RI, USA) were used to determine the electrical conductivity and pH of the juice. The TA was determined by titrating juice extract to pH 8.1 with 0.1 M NaOH and then measuring the percentage of citric acid in the samples (Piñero et al., 2020).

Data analysis
The variance of melon parameters was calculated using ANOVA. The means were compared using Duncan's multiple range test (DMRT) and differences were reported as significant at the p < 0.05 or p < 0.01 levels. The melon fruit traits were analyzed using a principal components analysis and the first two components were plotted. All statistical analyses were performed using the R software (version 3.6.1) with the Agricolae and Devtools packages (de Mendiburu, 2021;Wickham et al., 2021). Table 1 shows the analysis of variance of agronomic traits among the eight melon varieties. Most agronomic traits were significantly different (p < 0.05 or 0.01) between the melon varieties in all observed weeks, except for stem diameter at the first and third weeks after planting. Figure 1 illustrates melon growth based on agronomic traits. Five weeks after planting, V8 and V7 (1.02 ± 0.02 and 1.00 ± 0.03 cm) produced the biggest stem diameter and V7, V4, V3, and V1 (259.06 ± 7.15, 258.50 ± 7.04, 251.56 ± 6.62, 241.31 ± 5.08 cm) produced the longest stem length. V1 (27.38 ± 0.46 and 24.78 ± 0.41 cm) produced the widest and longest leaf and V2 (38.38 ± 0.52) produced the highest number of leaves. V8 (14.88 ± 0.43 days) had the earliest male flowers and V2, V5, V1, V3, and V7 (23.88 ± 0.29, 24.00 ± 0.43, 24.56 ± 0.27, 24.56 ± 0.42, 24.81 ± 0.61 days) had the earliest female flowers.  Table 2 shows the analysis of variance of melon fruit traits. All fruit traits were significantly different (at Table 1 Analysis of variance of melon agronomic traits.
p < 0.01) between the melon varieties in all observed weeks, except for the melon fruit cavity size. Table 2 shows that most fruit traits were significantly different between foliar fertilizer treatments (at p < 0.05 or 0.01) in most observed weeks except: fruit weight in the first week, fruit height in all observed weeks, fruit perimeter in the first and third weeks, fruit cavity, L Ã values, and c Ã values. Figure 3 illustrates melon growth, based on fruit traits. Six weeks after pollination, V7 (2.07 ± 0.09 kg) produced the heaviest fruit while the lowest fruit weight was obtained from V6 (1.00 ± 0.05 kg). V7, V4, V1, V2, and V5 (15.59 ± 0.20, 14.91 ± 0.23, 14.90 ± 0.16, 14.86 ± 0.26, 14.42 ± 0.14 cm) produced the longest fruit and V6 (12.00 ± 0.15 cm) produced the shortest fruit. V7, V5, and V4 (49.25 ± 0.77, 47.50 ± 0.65, 46.35 ± 0.54 cm) produced the fruit with the largest perimeter, and the smallest fruit perimeter was obtained from the fruit of V6 (39.90 ± 0.58 cm). Among foliar fertilizer applications at 6 weeks after pollination, melons treated with A+M (1.64 ± 0.13 kg and 45.69 ± 1.22 cm), S+M (1.59 ± 0.14 kg and 44.94 ± 1.26 cm) and M (1.50 ± 0.09 kg and 44.24 ± 0.79 cm) had larger fruit weights and fruit perimeters than melons under the non-foliar fertilizer application (control: C). There were no significant differences in fruit height between the four foliar fertilizer treatment groups. Table 3 shows the mean comparison among the melon fruit traits. Among the melon varieties, V7 produced the thickest (4.16 ± 0.18 cm), firmest (1.11 ± 0.04 N), and brightest (L Ã ; 73.46 ± 0.67) fruit pulp. V4 produced the thinnest fruit rind (0.58 ± 0.03 cm). V3 produced the highest juice EC (6,840.50 ± 113.32 µS cm −1 ), juice pH (7.08 ± 0.05), and yellowest (b Ã ) pulp (39.93 ± 0.38). V6, V1, V3, V4, and V2 (12.18 ± 0.25, 11.88 ± 0.35, 11.73 ± 0.65, 11.66 ± 0.40, and 11.51 ± 0.38 Brix) produced the sweetest fruit pulp, and V6 had the highest TA (0.23 ± 0.01% citric acid). Among the foliar fertilizer applications, Table 3 shows that melons treated with foliar fertilizers had better results than control melons for all fruit traits except for fruit cavity size, a Ã , and c Ã values, which were not significantly different. Melons treated with M, S+M, and A+M had the thickest (3.75 ± 0.08, 3.63 ± 0.08, 3.69 ± 0.10 cm) and firmest pulp (0.89 ± 0.03, 0.92 ± 0.04, 0.93 ± 0.04 N).  Figure 4 illustrates the biplot between the first two principal components of melon fruit quality under various foliar fertilizer applications. V7 is the largest cluster in the first principal component and dominates in L Ã , hue, and fruit cavity size, while V6 is the smallest cluster and dominates in TA. The fruits of V1-V4 dominate EC, TSS, a Ã , b Ã , and c Ã . Foliar fertilizer application is widely used as a technique to improve the fruit quality and productivity of horticultural crops (Santos, 2013). This type of direct application can help  to reduce the overall quantity of fertilizer needed in plant production while preserving fertilizer efficiency. Furthermore, foliar fertilizer application helps minimize the adverse consequences of excessive fertilizer usage, such as soil acidification, salinization, and The resistance and hardness of melon fruit skin are both influenced by calcium, which, as a molecular signaling agent, is a mineral that helps to strengthen plant cell walls and fruit skin (Cybulska, Zdunek & Konstankiewicz, 2011). Magnesium (Mg) is an important component of the chlorophyll molecule, which is necessary for photosynthesis and protein synthesis in plants. Mg has been associated with other elements in melon fruit, such as calcium, potassium, and manganese, according to several studies. Mg is also a cofactor for several enzymes involved in cell wall formation, including glutamine synthetase, xylose isomerase, and isocitrate lyase (Lopez-Zaplana et al., 2020). Iron is a necessary micronutrient for plant metabolism, including photosynthesis, respiration, DNA synthesis, and metabolic activators in a variety of pathways. It is a component of a number of key enzymes, including cytochromes, which are a crucial component of the electron transport chain in cellular respiration (Rout & Sahoo, 2015). Zinc is engaged in the ion transport mechanism in plant cells. Zinc maintains the balance between the phospholipid levels and membrane integrity, which affects plant water absorption and racking (Dang et al., 2010). Manganese also has an impact on various plant cell functions, such as amino acid synthesis and lignin biosynthesis. The hardness of the melon fruit peel is affected by low manganese levels, resulting in fruit cracking (Chen et al., 2016). Boron is required for the formation of new tissues and the production of cell walls. It has a relationship with the cell membrane's integrity and permeability. Higher calcium levels are produced by an increase in Boron in the melon pulp and rind (Lewis, 2019;Lopez-Zaplana et al., 2020).
Copper is an essential element of cellular physiological activities, such as energy generation. It is often used as a foliar fertilizer and stays on the leaf surface. Through antagonist and synergistic actions, excess copper has a negative impact on root metabolic activity and nutrient absorption (Torre, Iovino & Caradonia, 2018). Molybdenum is a trace element that plants need for development and for a variety of metabolic processes. It regulates the oxidation and reduction reactions of enzymes, and is an essential component of organic molecules known as molybdenum co-factors (Kaiser et al., 2005). Plant amino acids are precursors of a number of chemicals that are involved in cell activity. They are involved in the production of nitrogenous bases (purine and pyrimidine), alkaloids, and terpenoids. These compounds are required for pollination and fruit formation (Morsy, Abdel-Salam & Shams, 2018). In future investigations, other fruit quality attributes, such as vitamins and non-vitamin phytochemicals like flavonoids, should be included as they might also be affected by foliar fertilizer application.

CONCLUSIONS
Most agronomic traits were significantly different between the melon varieties in all observed growth weeks, except for stem diameter in the first and the third weeks after planting. All fruit traits were significantly different between the melon varieties in all observed growth periods. Sandee, Baramee, Melon cat 697, and Melon Princess are recommended for planting under Nakhon Si Thammarat's climate, based on fruit size and quality. Most fruit traits were significantly different between the different foliar fertilizer treatment groups in most observed growth weeks. The shape, skin color, and quality of the melon were all affected by foliar fertilizer application. Melons treated with micronutrients, secondary nutrients and micronutrients, and amino acids and micronutrients had higher measures of fruit quality than melons grown with non-foliar applications. There were also interactions observed between melon variety and foliar fertilizer application. Based on measures of fruit quality, Baramee, Melon cat 697, Kissme, and Melon Princess were more responsive to foliar fertilizer application than other melon varieties tested.